1. Field of the Invention
The present invention relates to a heterocyclic compound, a light-emitting element, a light-emitting device, an electronic device, and a lighting device.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting substance is interposed between a pair of electrodes. By applying voltage to this element, light emission from the light-emitting substance can be obtained.
Since such a light-emitting element is of self-light-emitting type, it is considered that the light-emitting element has advantages over a liquid crystal display in that visibility of pixels is high, backlight is not required, and so on and is therefore suitable as flat panel display elements. In addition, it is also a great advantage that the light-emitting element can be manufactured as a thin and lightweight element. Furthermore, very high speed response is also one of the features of such elements.
Furthermore, since such light-emitting elements can be formed in a film form, they make it possible to provide planar light emission. Therefore, large-area elements can be easily formed. This feature is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, light-emitting elements also have great potential as planar light sources applicable to lighting devices and the like.
Such light-emitting elements utilizing EL can be broadly classified according to whether the light-emitting substance is an organic compound or an inorganic compound. In the case of an organic EL element in which a layer containing an organic compound used as the light-emitting substance is provided between a pair of electrodes, application of a voltage to the light-emitting element causes injection of electrons from the cathode and holes from the anode into the layer containing the organic compound having a light-emitting property, and thus a current flows. Light is emitted when the carriers (electrons and holes) are recombined and the organic compound returns to the ground state from the excited state where both the electrons and the holes are generated in organic molecules with a light-emitting property.
In improving element characteristics of such a light-emitting element, there are a lot of problems which depend on a substance, and in order to solve the problems, improvement of an element structure, development of a substance, and the like have been carried out.
A light-emitting element using organic EL has a plurality of layers, and a carrier-transport layer is generally provided between a light-emitting layer and an electrode. One of the reasons is that a carrier-transport layer can prevent energy transfer of excitation energy from the light-emitting layer to the electrode and occurrence of quenching. Further, a material (an exciton-blocking material) having higher excitation energy than a light-emitting layer is preferably used for a carrier-transport layer which is adjacent to the light-emitting layer so that excitation energy is not transferred from the light-emitting layer. In other words, a material having a wide band gap (Bg) between the highest occupied molecular orbital level (HOMO level) and the lowest unoccupied molecular orbital level (LUMO level) is considered preferable.
In a light-emitting element using organic EL, a carrier-transport layer provided between a light-emitting layer and an electrode may include a plurality of layers. One possible reason is to adjust a carrier-injection barrier between adjacent layers. It can be considered that with a higher injection barrier, carrier passage can be suppressed and this leads to more efficient recombination in the light-emitting layer.
In the case of an element which emits phosphorescence, excitation energy of a light-emitting substance would be lost unless the level of triplet excitation energy (T1 level) of a material in contact with the light-emitting substance is sufficiently higher than the T1 level of the light-emitting substance. Therefore, as a host material of a light-emitting layer of a phosphorescent light-emitting element or a material of a carrier-transport layer adjacent to the light-emitting layer, a material having a T1 level higher than that of a phosphorescent light-emitting material is used.
However, many of common materials having a wide band gap or a high T1 level have low molecular weights so as not to extend conjugation. Due to their low molecular weights, these materials have many problems such as significantly poor thermophysical properties (a low glass transition temperature (Tg), a strong tendency toward crystallization) and poor film quality. Therefore, a material which can overcome these problems as well as having a wide band gap and a high T1 level is desired.
For example, Reference 1 discloses 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) as a material which can be used for a hole-transport layer of a light-emitting element.    [Reference 1] Chem. Mater., 1998, 10, pp. 2235-2250